The present invention basically relates to the general field of data unit communication. In data unit communication, an amount of data is divided into individual units, and said units are transmitted to a desired receiver over an appropriate communication path. This form of data communication is very well known and in wide use.
Such data units carry a variety of names in the context of different communication systems and communication protocols, such as packets, frames, segments, protocol data units, etc. The term “data unit” as used in the present specification and claims generically refers to any such division of a data amount.
In order to ensure the complete transmission of data units from a sender to a receiver, a mechanism referred to as ARQ (Automatic Retransmission reQuest) is known. When using an ARQ mechanism, the receiver of data units sends feedback messages to the sender, such that the sender can determine whether sent data units were properly received, and if not, to appropriately perform retransmissions of data units.
It can also occur that the communication of data units from a given sender to a given receiver occurs via one or more relay points. One example of such a situation is if a desk top computer communicates with a portable computer that has a WLAN module, where the communication is handled via a WLAN router. Another example of such a situation is if a link layer (layer 2) communication between a sender and receiver occurs over several relay points. Such connections are also referred to as multi-hop connections.
The basic problem encountered with such multi-hop connections is how to provide a reliable transmission of data units from the sender (i.e. the sending end-point) to the receiver (i.e. the receiving end-point). The paper “A comparison of Mechanisms for Improving TCP Performance over Wireless Links” by H. Balakrishnan et al, Proc. ACM SIGCOMM'96, Stanford, Calif., August 1996, gives an overview of techniques for dealing with multi-hop connections that involve wireless links.
One known solution to the multi-hop problem is the provision of split connections. An example of this principle is shown in FIG. 1. In the example of FIG. 1, a link layer (layer 2) communication between a sender 10 and receiver 12 via a relay device 11 is considered. In order to provide reliable transmission of layer 2 data units, a sending peer 10_2 in sender 10 and a receiving peer 11_2b in the relay device 11 implement an ARQ mechanism, and furthermore a sending peer 11_2a of relay device 11 and a receiving peer 12_2 of receiver 12 implement another ARQ mechanism. In this way, the first ARQ mechanism provides for reliability from sender 10 to relay device 11, and the second ARQ mechanism provides for reliability in the communication from relay device 11 to receiver 12. Naturally, this split connection concept is applicable to any layer, not just the link layer.
Nonetheless, it suffers from the disadvantage that if any problems occur in the relay device 11, or a handover from the shown relay device 11 to another relay device becomes necessary, then the entire end-to-end communication is in jeopardy. More specifically, it can occur that the sender 10 has completed its communication with the relay device 11, as the correct receipt of data units at relay 11 has been acknowledged to sender 10, and thereafter a problem occurs in relay device 11, such that some of these data units are lost. In such an event, these data units will be irrevocably lost at the given protocol level (L2 in the example), as the sender has already completed its communication and consequently deleted the data units from its send buffer.
In order to avoid such problems, it is known to introduce sub-layering. This is shown in an example in FIG. 2. The example again relates to a link layer communication between a sender 20 and a receiver 22. In order to provide end-to-end reliability, the link layer L2 is divided into two sub-layers, where the upper sub-layer has two peers 20_2′ and 22_2′ located at the sender 20 and receiver 22, respectively. These two peers implement their own ARQ mechanism, in order to provide for retransmission if data units of this upper sub-layer are lost on the end-to-end connection. Additionally, a lower sub-layer is provided, having respective peers 20_2 and 21_2b between sender 20 and relay device 21, and 21_2a and 22_2 between relay device 21 and receiver 22. The data units of the upper sub-layer are encapsulated or segmented into data units of the lower sub-layer, and each lower sub-layer peer pair has its own ARQ mechanism. Modifications of this sub-layering concept are known, e.g. U.S. Pat. No. 5,699,367 describes a situation, where the lower sub-layer is only provided on one hop, e.g. only between sender 20 and relay device 21.
Due to the end-to-end ARQ mechanism of peers 20_2′ and 22_2′, problems in the relay device 21 do not lead to irrevocable data unit loss. On the other hand, the ARQ mechanisms on the hops from sender to relay device and relay device to receiver ensure resource efficient and fast error recovery over each hop, e.g. by avoiding unnecessary end-to-end retransmissions. Nonetheless, the concept of sub-layering has the disadvantage of requiring complicated adjustment of the ARQ control in the upper sub-layer and lower sub-layer, in order to avoid ARQ conflicts, which can e.g. lead to unnecessary redundant data transmission, which in turn degrades the end-to-end performance.
WO 03/069837 A1 describes a method for retransmission of packets in a base station sub-system. A BSC communicates with an MS via a BTS, where the BSC and BTS are connected over a transport network. The BSC and the BTS use the GSL protocol on the physical layer. On the other hand, the BSC and the MS are RLC peers. It is disclosed to provide two types of messages between the BSC and the BTS: a first message format that comprises a data block, and a second format that only uses headers but'does not comprise a data block. In this way, the message of the second format identifies a data block, but does not carry the data block. Moreover, the BTS stores the data blocks that it successfully receives. The BTS also forwards these data blocks to the MS. If the MS does not receive a data block correctly, this is identified with the help of a NACK message sent in response to a polling signal. Two basic alternatives are described, where the first alternative is such that the BTS sends reception status messages in response to a polling signal, in order to inform the BSC. In this first alternative, the BTS passes the ACKs/NACKs sent by the MS to the BSC without any processing. The BSC does not need to send a message of the first format (with data block), as it is sufficient to send a message of the second format (without data block), such that the BTS can identify the missing data block and perform the retransmission, without having to again send the data block over the transport network.
On the other hand, if a reception status messages sent by the BTS to the BSC indicates a missing data block, then the BSC retransmits a message of the first format (i.e. with the missing data block). The BSC is controlled in such a way that if a NACK from the MS arrives, the BSC knows that this is due to an error in the transport network and has been corrected by an appropriate retransmission. After having received ACK messages from the MS, the BSC sends clear messages to the BTS, such that the BTS removes corresponding data blocks from its memory. In the second alternative, the BTS does not confirm a successful reception of data blocks, but only requests retransmissions of data blocks that have not been correctly received. In this second alternative the BTS must itself take care of the maintenance of its memory and it will not be the BSC that controls the removal of stored data blocks in the memory of the BTS. Namely, the BTS waits for acknowledgment messages 332 from the MS and interprets these messages. If a NACK is received from the MS, the BTS first checks whether it has the corresponding data block in its memory, and retransmits the data block if it is available. If it is not available, then the BTS requests the retransmission of the missing data block from the BSC. The BTS also forwards the NACK. In this way the BSC can respond in one of two ways: if it receives the NACK and a retransmission request, it sends a message of the first format (with data block), otherwise it sends a message of the second format (without the data block).